With the continuous progress of science and technology and the ever-growing need of liquid crystal display device, TFT-LCD (Thin Film Transistor-Liquid Crystal Display) has become the mainstream display used in the products such as cell-phone and panel computer, etc.
The performance of thin film transistor (TFT) determines the display quality of liquid crystal display. The existing TFT array substrate generally comprises in turn a base substrate, a common electrode, a gate electrode, a gate insulation layer, a semiconductor layer, an ohmic contact layer, a source electrode and a drain electrode, and a passivation layer and a pixel electrode, wherein the gate electrode, the source electrode and the drain electrode are usually made of metal with better electric conductivity. In order to improve the electric conductivity of metal electrodes, the metal electrodes can be made of Cu or Cu alloy. However Cu is a kind of metal susceptible to oxidation. When the metal electrode is made of Cu or Cu alloy, a superficial cupric oxide layer is apt to be formed on the metal electrode, and thus the impedance of the metal electrode will be increased and the electric conductivity decreased. Moreover, it will lead to a rough surface of the metal electrode and peeling off of the insulation layer costing the metal electrode. Therefore the performance of the TFT will be seriously affected, and the display will fail to display normally. If the metal electrode is made of other kinds of metal such as silver, aluminum or alloy, the metal electrode is also apt to be oxidized and the performance of TFT will be adversely affected.